Photoelectron Photoion Coincidence Technique Research Articles

Competitive Dehydrogenation and Backbone Fragmentation of Superhydrogenated PAHs: A Laboratory Study

Abstract Superhydrogenated polycyclic aromatic hydrocarbons (PAHs) have been suggested to catalyze the formation of H2 in certain regions of space, but it remains unclear under which circumstances this mechanism is viable given the reduced carbon backbone stability of superhydrogenated PAHs. We report a laboratory study on the stability of the smallest pericondensed PAH, pyrene (C16H10+N , with N = 4, 6, and 16 additional H atoms), against photodestruction by single vacuum ultraviolet photons using the photoelectron–photoion coincidence technique. For N = 4, we observe a protective effect of hydrogenation against the loss of native hydrogens, in the form of an increase in the appearance energies of the and C16H8 + daughter ions compared to those reported for pristine pyrene (C16H10). No such effect is seen for N = 6 or 16, where the weakening effect of replacing aromatic bonds with aliphatic ones outweighs the buffering effect of the additional hydrogen atoms. The onset of fragmentation occurs at similar internal energies for N = 4 and 6, but is significantly lower for N = 16. In all three cases, H-loss and C m H n -loss (m ≥ 1, carbon backbone fragmentation) channels open at approximately the same energy. The branching fractions of the primary channels favor H-loss for N = 4, C m H n -loss for N = 16, and are roughly equal for the intermediate N = 6. We conclude that superhydrogenated pyrene is probably too small to support catalytic H2-formation, while trends in the current and previously reported data suggest that larger PAHs may serve as catalysts up to a certain level of hydrogenation.

The Underlying Chemistry to the Formation of PO2 Radicals from Organophosphorus Compounds: A Missing Puzzle Piece in Flame Chemistry.

Reactive species, such as . PO2 and HOPO, are considered of upmost importance in flame inhibition and catalytic combustion processes of fuels. However, the underlying chemistry of their formation remains speculative due to the unavailability of suitable analytical techniques that can be used to identify the transient species which lead to their formation. This study elucidates the reaction mechanisms of the formation of phosphoryl species from dimethyl methyl phosphonate (DMMP) and dimethyl methyl phosphoramidate (DMPR) under well-defined oxidative conditions. Photoelectron photoion coincidence techniques that utilized vacuum ultraviolet synchrotron radiation were applied to isomer-selectively detect the elusive key intermediates and stable products. With the help of in situ recorded spectral fingerprints, different transient species, such as PO2 and triplet O radicals, have been exclusively identified from their isomeric components, which has helped to piece together the formation mechanisms of phosphoryl species under various conditions. It was found that . PO2 formation required oxidative conditions above 1070 K. The combined presence of O2 and H2 led to significant changes in the decomposition chemistry of both model phosphorus compounds, leading to the formation of . PO2 . The reaction . PO+O2 →. PO2 +O: was identified as the key step in the formation of . PO2 . Interestingly, the presence of O2 in DMPR thermolysis suppresses the formation of PN-containing species. In a previous study, PN species were identified as the major species formed during the pyrolysis of DMPR. Thus, the findings of this study has shed light onto the decomposition pathways of organophosphorus compounds, which are beneficial for their fuel additive and fire suppressant applications.

Photoabsorption and Photoionization Cross Sections of Pyridine in the Vacuum-Ultraviolet Energy Range.

We have performed an experimental investigation into the interaction of vacuum-ultraviolet synchrotron radiation with pyridine molecules in the gas phase. Specifically, a double-ion chamber spectrometer was used to measure the absolute photoabsorption cross sections and the photoionization quantum yields from the ionization threshold to 21.5 eV. Moreover, photoionization and neutral-decay cross sections in absolute scale were derived from these data. In addition, the fragmentation pattern was investigated as a function of the photon energy by using a time-of-flight mass spectrometer and the photoelectron-photoion coincidence technique. Thus, the absolute partial ionization cross sections for each ionic fragment were obtained. Comparisons are made with experimental data available in the literature.

New insights onto dissociation of state-selected O2+ ions investigated by double imaging photoelectron photoion coincidence: The superimposed 32Πu and c4Σu- inner-valence states.

The double imaging photoelectron photoion coincidence technique has been applied to investigate the dissociation of state-selected O2+(32Πu, c4Σu-) ions at hv = 25.4 eV. It is shown that the O2+ ions prepared in the 32Πu inner-valence state can dissociate toward two channels where a distinct changeover from the third dissociation limit O(3P) + O+(2D) to the fifth limit O(3P) + O+(2P) is observed at a binding energy of ∼23.8 eV. Up to four dissociation limits have been identified in the dissociation of the v = 0 vibrational level of the c4Σu- electronic state, and in particular, the products toward the third limit with a very weak intensity are definitely identified and assigned. In addition, the individual contributions of the two superimposed 32Πu and c4Σu- states to the fifth dissociation limit are clearly separated and determined for the first time. The anisotropy parameters of the O+ fragment ion toward different dissociation limits have also been measured and exhibit specific state-selected characteristics. The dissociation mechanisms of the 32Πu and c4Σu- states including predissociation and tunneling motion are proposed based on the present experimental results and the existing theoretical potential energy curves.

Photoabsorption and photoionization cross sections for formaldehyde in the vacuum-ultraviolet energy range

We report a theoretical-experimental investigation on the interaction of vacuum-ultraviolet radiation with formaldehyde (H2CO) in the gas phase. Experimentally, the absolute photoabsorption cross sections and the photoionization quantum yields were measured in the (11.0–21.5) eV range using the double-ion chamber technique. Also, the absolute photoionization and neutral-decay cross sections were derived from these data. In addition, in the same energy region, the dissociation pattern was obtained with a time-of-flight mass spectrometer using the photoelectron-photoion coincidence technique, and the absolute photoionization cross sections were derived for each ionic fragment observed. Moreover, theoretical photoionization cross sections were calculated for the ionization of the four outermost molecular valence orbitals (2b2, 1b1, 5a1, and 1b2) from the threshold to 35 eV. The calculations were performed using the iterative Schwinger variational method to solve the Lippmann-Schwinger equation in the exact static-exchange level of approximation. In general, there is a good agreement between our experimental and previous data reported in the literature. Our theoretical results show a fair qualitative agreement with the experimental data and with previous theoretical results. Above 20 eV, a better quantitative agreement with the experimental data is also observed.

Photoionisation study of Xe.CF4 and Kr.CF4 van-der-Waals molecules

We report on photoionization studies of Xe.CF4 and Kr.CF4 van-der-Waals complexes produced in a supersonic expansion and detected using synchrotron radiation and photoelectron-photoion coincidence techniques. The ionization potential of CF4 is larger than those of the Xe and Kr atoms and the ground state of the Rg.CF4 (+) ion correlates with Rg(+) ((2)P3/2) + CF4. The onset of the Rg.CF4 (+) signals was found to be only ∼0.2 eV below the Rg ionization potential. In agreement with experiment, complementary ab initio calculations show that vertical transitions originating from the potential minimum of the ground state of Rg.CF4 terminate at a part of the potential energy surfaces of Rg.CF4 (+), which are approximately 0.05 eV below the Rg(+) ((2)P3/2) + CF4 dissociation limit. In contrast to the neutral complexes, which are most stable in the face geometry, for the Rg.CF4 (+) ions, the calculations show that the minimum of the potential energy surface is in the vertex geometry. Experiments which have been performed only with Xe.CF4 revealed no Xe.CF4 (+) signal above the first ionization threshold of Xe, suggesting that the Rg.CF4 (+) ions are not stable above the first dissociation limit.

Ionization and Fragmentation of Formamide Induced by Synchrotron Radiation in the Valence Region via Photoelectron Photoion Coincidence Measurements and Density Functional Theory Calculations.

We have performed a theoretical and experimental study of the formamide (HCONH2) photofragmentation and photoionization processes in the gas phase. The experiment was perfomed by using a time-of-flight mass spectrometer using the photoelectron photoion coincidence (PEPICO) technique in the valence region, from photons with energy between 10 and 20 eV. We have obtained both mass and partial ion yield spectra, identified by the mass-to-charge ratio as a function of the photon energy. With this setup, we could ascertain the threshold energy for the production of formamide cation and its cationic fragments. The theoretical analysis of the formamide photofragmentation channels are fulfilled by the density functional theory (DFT) and the time-dependent density functional theory (TDDFT). The theoretical analysis allowed us to estimate, for example, which atoms are lost during the photofragmentation. We have also developed a theoretical-experimental analysis of the main fragments produced in the dissociation: m/q = 45 (HCONH2+), m/q = 44 (CONH2+), m/q = 29 (HCO+), m/q = 17 (NH3+), and m/q = 16 (NH2+).

Fragmentation of mercury compounds under ultraviolet light irradiation.

Ultraviolet light induced photofragmentation of mercury compounds is studied experimentally with electron energy resolved photoelectron-photoion coincidence techniques and theoretically with computational quantum chemical methods. A high resolution photoelectron spectrum using synchrotron radiation is presented. Fragmentation of the molecule is studied subsequent to ionization to the atomic-mercury-like d orbitals. State dependent fragmentation behaviour is presented and specific reactions for dissociation pathways are given. The fragmentation is found to differ distinctly in similar orbitals of different mercury compounds.

Theoretical and Experimental Photoelectron Spectroscopy Characterization of the Ground State of Thymine Cation.

We report on the vibronic structure of the ground state X̃(2)A″ of the thymine cation, which has been measured using a threshold photoelectron photoion coincidence technique and vacuum ultraviolet synchrotron radiation. The threshold photoelectron spectrum, recorded over ∼0.7 eV above the ionization potential (i.e., covering the whole ground state of the cation) shows rich vibrational structure that has been assigned with the help of calculated anharmonic modes of the ground electronic cation state at the PBE0/aug-cc-pVDZ level of theory. The adiabatic ionization energy has been experimentally determined as AIE = 8.913 ± 0.005 eV, in very good agreement with previous high resolution results. The corresponding theoretical value of AIE = 8.917 eV has been calculated in this work with the explicitly correlated method/basis set (R)CCSD(T)-F12/cc-pVTZ-F12, which validates the theoretical approach and benchmarks its accuracy for future studies of medium-sized biological molecules.

Excited state dynamics in SO2. I. Bound state relaxation studied by time-resolved photoelectron-photoion coincidence spectroscopy.

The excited state dynamics of isolated sulfur dioxide molecules have been investigated using the time-resolved photoelectron spectroscopy and time-resolved photoelectron-photoion coincidence techniques. Excited state wavepackets were prepared in the spectroscopically complex, electronically mixed (B̃)(1)B1/(Ã)(1)A2, Clements manifold following broadband excitation at a range of photon energies between 4.03 eV and 4.28 eV (308 nm and 290 nm, respectively). The resulting wavepacket dynamics were monitored using a multiphoton ionisation probe. The extensive literature associated with the Clements bands has been summarised and a detailed time domain description of the ultrafast relaxation pathways occurring from the optically bright (B̃)(1)B1 diabatic state is presented. Signatures of the oscillatory motion on the (B̃)(1)B1/(Ã)(1)A2 lower adiabatic surface responsible for the Clements band structure were observed. The recorded spectra also indicate that a component of the excited state wavepacket undergoes intersystem crossing from the Clements manifold to the underlying triplet states on a sub-picosecond time scale. Photoelectron signal growth time constants have been predominantly associated with intersystem crossing to the (c̃)(3)B2 state and were measured to vary between 750 and 150 fs over the implemented pump photon energy range. Additionally, pump beam intensity studies were performed. These experiments highlighted parallel relaxation processes that occurred at the one- and two-pump-photon levels of excitation on similar time scales, obscuring the Clements band dynamics when high pump beam intensities were implemented. Hence, the Clements band dynamics may be difficult to disentangle from higher order processes when ultrashort laser pulses and less-differential probe techniques are implemented.

Ionization photophysics and spectroscopy of cyanoacetylene.

Photoionization of cyanoacetylene was studied using synchrotron radiation over the non-dissociative ionization excitation range 11-15.6 eV, with photoelectron-photoion coincidence techniques. The absolute ionization cross-section and spectroscopic aspects of the parent ion were recorded. The adiabatic ionization energy of cyanoacetylene was measured as 11.573 ± 0.010 eV. A detailed analysis of photoelectron spectra of HC3N involves new aspects and new assignments of the vibrational components to excitation of the A(2)Σ(+) and B(2)Π states of the cation. Some of the structured autoionization features observed in the 11.94 to 15.5 eV region of the total ion yield (TIY) spectrum were assigned to two Rydberg series converging to the B(2)Π state of HC3N(+). A number of the measured TIY features are suggested to be vibrational components of Rydberg series converging to the C(2)Σ(+) state of HC3N(+) at ≈17.6 eV and others to valence shell transitions of cyanoacetylene in the 11.6-15 eV region. The results of quantum chemical calculations of the cation electronic state geometries, vibrational frequencies and energies, as well as of the C-H dissociation potential energy profiles of the ground and electronic excited states of the ion, are compared with experimental observations. Ionization quantum yields are evaluated and discussed and the problem of adequate calibration of photoionization cross-sections is raised.

Ionization photophysics and spectroscopy of dicyanoacetylene

Photoionization of dicyanoacetylene was studied using synchrotron radiation over the excitation range 8-25 eV, with photoelectron-photoion coincidence techniques. The absolute ionization cross-section and detailed spectroscopic aspects of the parent ion were recorded. The adiabatic ionization energy of dicyanoacetylene was measured as 11.80 ± 0.01 eV. A detailed analysis of the cation spectroscopy involves new aspects and new assignments of the vibrational components to excitation of the quasi-degenerate A(2)Πg, B(2)Σg(+) states as well as the C(2)Σu(+) and D(2)Πu states of the cation. Some of the structured autoionization features observed in the 12.4-15 eV region of the total ion yield spectrum were assigned to vibrational components of valence shell transitions and to two previously unknown Rydberg series converging to the D(2)Πu state of C4N2(+). The appearance energies of the fragment ions C4N(+), C3N(+), C4(+), C2N(+), and C2(+) were measured and their heats of formation were determined and compared with existing literature values. Thermochemical calculations of the appearance potentials of these and other weaker ions were used to infer aspects of dissociative ionization pathways.

Comparing Femtosecond Multiphoton Dissociative Ionization of Tetrathiafulvene with Imaging Photoelectron Photoion Coincidence Spectroscopy

In this paper we describe femtosecond photoionization and the imaging photoelectron photoion coincidence spectroscopy of tetrathiafulvene, TTF. Femtosecond photoionization of TTF results in the absorption of up to twelve 808 nm photons leading to ion internal energies up to 12.1 eV as deduced from the photoelectron spectrum. Within this internal energy a variety of dissociation channels are accessible. In order to disentangle the complex ionic dissociation, we utilized the imaging photoelectron photoion coincidence (iPEPICO) technique. Above the dissociation threshold, iPEPICO results show that the molecular ion (m/z = 204) dissociates into seven product ions, six of which compete in a 1.0 eV internal energy window and are formed with the same appearance energy. Ab initio calculations are reported on the possible fragment ion structures of five dissociation channels as well as trajectories showing the loss of C2H2 and C2H2S from high internal energy TTF cations. A three-channel dissociation model is used to fit the PEPICO data in which two dissociation channels are treated as simple dissociations (one with a reverse barrier), while the rest involve a shared barrier. The two lower energy dissociation channels, m/z = 146 and the channel leading to m/z = 178, 171, 159, 140, and 127, have E0 values of 2.77 ± 0.10 and 2.38 ± 0.10 eV, respectively, and are characterized by ΔS(‡)(600 K) values of -9 ± 6 and 1 ± 6 J K(-1) mol(-1), respectively. Competing with them at higher internal energy is the cleavage of the central bond to form the m/z = 102 fragment ion, with an E0 value of 3.65 ± 0.10 eV and ΔS(‡)(600 K) = 83 ± 10 J K(-1) mol(-1).

A dissociative photoionization study of the c4Σu− state in O+2 using the TPEPICO technique

In the dissociative photoionization (DPI) process, hν + O2 → O + O+ + e−, ionisation and dissociation both occur (either as a direct or indirect process) following photoabsorption. The Threshold Photoelectron Photoion Coincidence (TPEPICO) technique, i.e. measuring the coincidence yield between threshold photoelec-trons and photoions is a powerful way of studying the dynamics involved. The c4Σu− state in O+2 at ~ 24.56 eV has a shallow minimum in its potential that supports two distinct quasi-bound vibrational levels (v = 0, 1). We have investigated the angular distributions of 2 eV O+(4S) ions produced from DPI of O +2 c4Σu− (v = 0, 1) using the TPEPICO technique.

Size selective spectroscopy of Se microclusters

The electronic structure and photofragmentation in outer and inner valence regions of Se(n) (n ≤ 8) clusters produced by direct vacuum evaporation have been studied with size-selective photoelectron-photoion coincidence technique by using vacuum-ultraviolet synchrotron radiation. The experimental ionization potentials of these clusters were extracted from the partial ion yield measurements. The calculations for the possible geometrical structures of the Se(n) microclusters have been executed. The ionization energies of the clusters have been calculated and compared with the experimental results. In addition, theoretical fragment ion appearance energies were estimated. The dissociation energies of Se(n) clusters were derived from the recurrent relation between the gas phase enthalpies of the formation of corresponding cationic clusters and experimental ionization energies.

Formation of Kr3+via core-valence doubly ionized intermediate states

The time-of-flight photoelectron-photoion coincidence technique has been used to study single-photon $3{d}^{9}4{p}^{5}$ core-valence double ionization of Kr and subsequent Auger decay to triply charged states associated with the $4{s}^{2}4{p}^{3}$ and $4{s}^{1}4{p}^{4}$ configurations. The photon energy used was $h\ensuremath{\nu}=150$ eV. Multiconfiguration Dirac-Fock calculations were performed both for the doubly ionized intermediate states and the triply ionized final states. The intermediate states of Kr${}^{2+}$ are observed between 120 and 125 eV, whereas the final states of Kr${}^{3+}$ are observed between 74- and 120-eV ionization energy. Assignments of all structures are made based on the present numerical results. The calculated Auger rates give a detailed explanation of the relative line strengths observed.

Threshold Photoelectron Spectroscopy of Cyclopropenylidene, Chlorocyclopropenylidene, and Their Deuterated Isotopomeres

Cyclopropenylidene (c-C(3)H(2)), chlorocyclopropenylidene (c-C(3)HCl), and their deuterated isotopomers were studied by the threshold photoelectron-photoion coincidence (TPEPICO) technique using VUV synchrotron radiation. The carbenes were generated via flash pyrolysis. In all species a change in geometry is visible upon ionization, with significant activity in the C═C, C-C-stretching mode and, in the case of c-C(3)H(2)/D(2), the C-H-bending mode. The electron is removed from an sp(2) like hybrid orbital centered on the carbene C atom. The mass selected threshold photoelectron (TPE) spectra were fitted by a Franck-Condon simulation, yielding the equilibrium geometry of the cation ground state ((1)A(1)). The adiabatic ionization energy IE(ad) of c-C(3)H(2) was determined to be 9.17 eV, in good agreement with calculations and literature values. Two vibrational wavenumbers of the cation were determined experimentally (ν(3)(+) = 1150 cm(-1) and ν(2)(+) = 1530 cm(-1)). Chlorocyclopropenylidene was also studied by TPE spectroscopy and has a similar IE(ad) of 9.17 eV. The spectrum also shows a vibrational progression that corresponds to the C═C- and C-C-stretching modes of the cation. The equilibrium geometry was also determined by a Franck-Condon fit. The IE(ad) of the deuterated isotopomers, c-C(3)D(2) and c-C(3)DCl, were also determined to be 9.17 eV. The spectra confirm the assignments for the nondeuterated species.

VUV-induced dissociation of methylchlorosilanes, studied by electron–ion coincidence spectroscopy

Fragmentation patterns of gas-phase trimethylchlorosilane and methyltrichlorosilane molecules were investigated using the electron-energy-resolved photoelectron–photoion coincidence technique. The production of parent ions and the dominant fragments by 30 eV synchrotron radiation was followed as a function of the photoelectron binding energy. Ion appearance energies were determined from these coincident ion yield curves. The fragmentation patterns and their development are discussed in terms of available internal excess energy as well as the molecular orbital character of the parent molecules. A common dissociation scenario is proposed for both molecules, producing methyl- and chlorine-loss fragments at lower excess energies and smaller fragments through second-step processes at sufficiently high energies.

Photoion mass spectroscopy and valence photoionization of hypoxanthine, xanthine and caffeine

Photoionization mass spectra of hypoxanthine, xanthine and caffeine were measured using the photoelectron–photoion coincidence technique and noble gas resonance radiation at energies from 8.4 to 21.2 eV for ionization. The fragmentation patterns for these compounds show that hydrogen cyanide is the main neutral loss species at higher photon energies, while photoionization below 16.67 eV led predominantly to the parent ion. The valence photoelectron spectra of this family of molecules were measured over an extended energy range, including the inner C, N and O 2s valence orbitals. The observed ion fragments were related to ionization of the valence orbitals.

Electronic state resolved PEPICO spectroscopy of pyrimidine

Photoionization techniques, such as photoelectron spectroscopy (PES) and photoionization mass spectrometry (PIMS), are well-established and powerful methods for studying the spectroscopy of isolated bio-organic molecules and their fate under vacuum ultraviolet (VUV) irradiation. Measuring the energy selected electron leaving a molecular ion in coincidence with other particles, such as ions, can provide even deeper insights into the mechanisms of the interaction of molecules with ionizing radiation. We have thus implemented the electronic state resolved photoelectron photoIon coincidence (ER-PEPICO) technique in our laboratory. Here, we report our newly constructed apparatus, and its application for characterizing fragmentation processes occurring in pyrimidine. Ionization of the two highest molecular orbitals (MOs) of the valence band does not lead to fragmentation of the resulting ion. The third band observed in photoemission is due to the ionization of two MOs, and leads mainly to the formation of the parent ion. The next three electronic states are not resolved experimentally and appear as a single band; their ionization leads to fragments of mass to charge ratio m/e= 53 (C3H3N+), while ionization of deeper lying MOs leads mostly to m/e= 26 (C2H2+). We compare our data with previous non-coincidence photoionization results and describe the problems encountered and their solutions.